1. Field of the Invention
This invention relates to a silver containing copper alloy. More particularly, the inclusion of a controlled amount of silver in a copper alloy that further contains chromium, titanium and silicon results in improved resistance to stress relaxation and improved isotropic bend properties without a detrimental effect on either yield strength or electrical conductivity.
2. Description of Related Art
Copper alloys are formed into numerous products that take advantage of the high electrical conductivity and/or high thermal conductivity of the alloys. A partial list of such products includes electrical connectors, leadframes, wires, tubes, foils and powders that may be compacted into products. One type of electrical connector is a box-like structure formed by stamping a predefined shape from a copper alloy strip and then bending the stamped part to form the connector. It is necessary for the connector to have high strength and high electrical conductivity. In addition, the connector should have a minimal reduction in normal force as a function of time and temperature exposure, commonly referred to as resistance to stress relaxation.
Properties important for an electrical connector include yield strength, bend formability, resistance to stress relaxation, modulus of elasticity, ultimate tensile strength and electrical conductivity.
Target values for these properties and relative importance of the properties are dependent on the intended application of products manufactured from the subject copper alloys. The following property descriptions are generic for many intended applications, but the target values are specific for under the hood automotive applications.
The yield strength is the stress at which a material exhibits a specified deviation, typically an offset of 0.2%, from proportionality of stress and strain. This is indicative of the stress at which plastic deformation becomes dominant with respect to elastic deformation. It is desirable for copper alloys utilized as connectors to have a yield strength on the order of 80 ksi, that is approximately 550 MPa.
Stress relaxation becomes apparent when an external stress is applied to a metallic strip in service, such as when the strip is loaded after having been bent into a connector. The metal reacts by developing an equal and opposite internal stress. If the metal is held in a strained position, the internal stress will decrease as a function of both time and temperature. This phenomenon occurs because of the conversion of elastic strain in the metal to plastic, or permanent strain, by microplastic flow.
Copper based electrical connectors must maintain above a threshold contact force on a mating member for prolonged times for good electrical connection. Stress relaxation reduces the contact force to below the threshold leading to an open circuit. It is a target for a copper alloy for connector applications to maintain at least 90% of the initial stress when exposed to a temperature of 150xc2x0 C. for 1000 hours and to maintain 85% of the initial stress when exposed to a temperature of 200xc2x0 C. for 1000 hours.
The modulus of elasticity, also known as Young""s modulus, is a measure of the rigidity or stiffness of a metal and is the ratio of stress to corresponding strain in the elastic region. Since the modulus of elasticity is a measure of the stiffness of a material, a high modulus, on the order of 150 GPa is desirable.
Bendability determines the minimum bend radius (MBR) which identifies how severe a bend may be formed in a metallic strip without fracture along an outside radius of the bend. The MBR is an important property for connectors where different shapes are to be formed with bends at various angles.
Bend formability may be expressed as, MBR/t, where t is the thickness of the metal strip. MBR/t is a ratio of the minimum radius of curvature of a mandrel about which the metallic strip can be bent without failure. The xe2x80x9cmandrelxe2x80x9d test is specified in ASTM (American Society for Testing and Materials) designation E290-92, entitled Standard Test Method for Semi-Guided Bend Test for Ductility of Metallic Materials, and is incorporated by reference in its entirety herein.
It is desirable for the MBR/t to be substantially isotropic, a similar value in the xe2x80x9cgood wayxe2x80x9d, bend axis perpendicular to the rolling direction of the metallic strip, as well as the xe2x80x9cbad wayxe2x80x9d, bend axis parallel to the rolling direction of the metallic strip. It is desirable for the MBR/t to be about 0.5 or less for a 90xc2x0 bend and about 1 or less for a 180xc2x0 bend.
Alternatively, the bend formability for a 90xc2x0 bend may be evaluated utilizing a block having a V-shaped recess and a punch with a working surface having a desired radius. In the xe2x80x9cV-blockxe2x80x9d method, a strip of the copper alloy in the temper to be tested is disposed between the block and the punch and when the punch is driven down into the recess, the desired bend is formed in the strip.
Related to the V-block method is the 180xc2x0 xe2x80x9cform punchxe2x80x9d method in which a punch with a cylindrical working surface is used to shape a strip of copper alloy into a 180xc2x0 bend.
Both the V-block method and the form punch method are specified in ASTM designation B820-98, entitled Standard Test Method for Bend Test for Formability of Copper Alloy Spring Material, that is incorporated by reference in its entirety herein.
For a given metal sample, both methods give quantifiable bendability results and either method may be utilized to determine relative bendability.
The ultimate tensile strength is a ratio of the maximum load a strip withstands until failure during a tensile test expressed as a ratio of the maximum load to the cross-sectional area of the strip. It is desirable for the ultimate tensile strength to be about 85-90 ksi, that is approximately 585-620 MPa.
Electrical conductivity is expressed in % IACS (International Annealed Copper Standard) in which unalloyed copper is defined as having an electrical conductivity of 100% IACS at 20xc2x0 C. It is desirable for copper alloys for high performance electrical connectors to have an electrical conductivity of at least 75% IACS. More preferably, the electrical conductivity is 80% IACS or higher.
One copper alloy that approaches the desired properties is designated by the Copper Development Association, New York, N.Y., as C18600. C18600 is an iron containing copper-chromium-zirconium alloy and is disclosed in U.S. Pat. No. 5,370,840 to Caron, et al that is incorporated by reference in its entirety herein. C18600 has a nominal composition by weight of 0.3% chromium, 0.2% zirconium, 0.5% iron, 0.2% titanium and the balance copper and inevitable impurities.
Throughout this patent application, all percentages are expressed as weight percent unless otherwise noted.
Mechanical and electrical properties of copper alloys are highly dependent on processing. If C18600 is subjected to an aging anneal, a 33% cold roll and a relief anneal, the alloy achieves as nominal properties: an electrical conductivity of 73% IACS; a yield strength of 90 ksi; a 90xc2x0 MBR/t of 1.2 in the good way and 3.5 in the bad way utilizing the mandrel method(xe2x80x9croller bendxe2x80x9d method); and a 20% loss in stress when subjected to 200xc2x0 C. for 1000 hours.
U.S. Pat. No. 4,678,637 to Duerrschnabel et al discloses a copper alloy containing additions of chromium, titanium and silicon and is incorporated by reference in its entirety herein. This alloy, designated by the CDA as C18070, has a nominal composition of 0.28% chromium, 0.06% titanium, 0.04% silicon and the balance copper and unavoidable impurities. When processed by hot rolling, quench and cold rolling interspersed with one or two intermediate bell anneals, the alloy achieves as nominal properties: an electrical conductivity of 86% IACS; a yield strength of 72 ksi (496 MPa), a 90% MBR of 1.6t in the good way and 2.6t in the bad way; and a loss of 32% of the stress when subjected to 200xc2x0 C. for 1000 hours.
DE 196 00 864 C2 by Wieland-Werke AG, discloses an alloy containing 0.1%-0.5% chromium, 0.01%-0.25% titanium, 0.01%-0.1% silicon, 0.02%-0.8% magnesium with the balance being copper and inevitable impurities. The magnesium addition is disclosed as improving the resistance of the alloy to stress relaxation.
A small addition of silver, on the order of up to 25 troy ounces per ton avoirdupois (.085 weight percent), enables cold worked copper to maintain its strength at temperatures of up to about 400xc2x0 C. as disclosed in Silver-Bearing Copper by Finlay, 1968. One silver-containing copper alloy is designated by the CDA as copper alloy C15500. C15500 contains 0.027-0.10% silver, 0.04-0.08% phosphorous, 0.08-0.13% magnesium and the balance is copper and unavoidable impurities. The alloy is reported in the ASM Handbook as having an electrical conductivity of 90% IACS in an annealed condition, a yield strength of 72 ksi (496 MPa) in the spring temper. Bend formability and resistance to stress relaxation are not reported.
While the copper alloys described above achieve some of the desired properties for connectors, there remains a need for an improved copper alloy that comes closer to the target requirements and further, there remains a need to characterize a copper alloy utilizing a holistic system that integrates multiple customer identified desired properties into a single performance indicator.
Accordingly, it is an object of the invention to provide a copper base alloy that is particularly suited for electrical connector applications. It is a feature of the invention that this copper base alloy contains chromium, titanium and silver. Yet another feature of the invention is that iron and tin may be added to promote grain refinement and increase strength. Still another feature of the invention is to maximize desired electrical and mechanical properties by processing of the alloy including the steps of solution anneal, quench, cold roll and age. Still a further feature of the invention is that a holistic approach to alloy properties is utilized to integrate multiple alloy properties by way of factors weighted by customer derived rankings for specific connector applications.
It is an advantage of the invention that the alloy of the invention may be processed to have a yield strength in excess of 80 ksi (550 MPa) and an electrical conductivity in excess of 80% IACS making the alloy particularly useful for forming into electrical connectors for both automotive and multimedia applications. Among the advantageous properties of the alloy of the invention are an enhanced resistance to stress relaxation at elevated temperatures of up to 200xc2x0 C. A still further advantage is that a strip of metal formed from the alloy has substantially isotropic bend formability and excellent stampability making it particularly useful for forming into box-type connectors.
In accordance with the invention, there is provided a copper alloy that consists essentially of, by weight, from 0.15% to 0.7% of chromium, from 0.005% to 0.3% of silver, from 0.01% to 0.15% of titanium, from 0.01% to 0.10% of silicon, up to 0.2% of iron, up to 0.5% of tin, and the balance is copper and inevitable impurities.
In accordance with the invention there is provided a process for forming a copper alloy having high electrical conductivity, good resistance to stress relaxation and isotropic bend properties. This process includes the steps of casting a copper alloy that contains, by weight, from 0.15% to 0.7% of chromium, additional desired alloying additions, and the balance is copper and inevitable impurities. This copper alloy is formed into a strip that is solution annealed, by a strip anneal process, at a temperature of from 850xc2x0 C. to 1030xc2x0 C. for from 5 seconds to 10 minutes. A preferred strip anneal time is from 10 seconds to 5 minutes. The strip is then quenched from a temperature of at least 850xc2x0 C. to a temperature of less than 500xc2x0 C. in at most 10 seconds. The quenched strip is then cold rolled to a reduction of from 40% to 99% in thickness and then annealed at a temperature of between 350xc2x0 C. and 550xc2x0 C. for from one hour to 10 hours.
The above stated objects, features and advantages will become more apparent from the specification and drawings that follow.